It is commercially desirable to provide in the most economical way an assembly comprising a helical coil of strand, such as wire, with the coil convolutions separated from each other and positively held against displacement.
An example is a coil of electric resistance heating wire, such as might be used inside of a metal or refractory tube for heating the tube to form a radiant heater for use inside of industrial furnaces. Such a heater is normally required to use electric resistance wire of relatively heavy gauge, coiled in such a way that the coil convolutions are positively anchored in interspaced relationship, with the materials and manufacturing costs held at a minimum compatible with reliability.
The usual way for accomplishing this objective is to provide a refractory core that is helically grooved inwardly with the wire convolutions seated in these grooves. This is the accepted commercial practice, although other expedients have been proposed. There are objections to this practice.
One objection is that to fabricate the assembly, the wire or rod must be coiled with the convolutions separated and with the helical core grooves matching in pitch, the core then being screwed into the coil. This is objectionable from the manufacturing viewpoint concerning labor costs.
Another objection is that the refractory core must be a high-purity refractory such as substantially pure alumina, because otherwise the coil will chemically react with the electric resistance wire having the compositions often used. Such high-purity material is expensive.